1. Field of the Invention
The present invention is broadly concerned with improved, nonionic surfactant-based aqueous lubricants adapted for lubricating equipment designed to handle thermoplastic articles subject to stress cracking (e.g., polyethylene terephthalate (PET) containers). More particularly, the invention pertains to such lubricants in concentrate or use dilution form, and to methods for lubricating handling equipment, wherein the lubricant compositions are essentially free of oil and include a nonionic surfactant dispersed in water; the surfactant has a molecular weight of at least about 1000 and includes at least about 12 ethylene oxide moieties therein. Moreover, the surfactant makes up at least about 50% by weight of the non-aqueous components of the lubricant. Lubricants in accordance with the invention have been shown to have lubricity ratio and crazing values rendering them particularly suitable for use with conveyors and other handling equipment for PET containers.
2. Description of the Prior Art
Thermoplastic food and beverage containers are transported between cleaning, labeling, filling and packaging stations by conveyors and related equipment. In order to keep the conveyors clean and lubricated, and to facilitate handling of the containers, aqueous lubricants are conventionally sprayed onto the conveyors. Generally, the lubricants are supplied as concentrates and are diluted by the end user. In order to be successful, the aqueous lubricants must provide a lubricating function and should also facilitate cleaning and removal of food or beverage spills. Moreover, they must be compatible with tap water used as a diluent. A variety of materials have been used in the formulation of prior conveyor lubricants including fatty acid soaps (U.S. Pat. No. 3,860,521), phosphate esters (U.S. Pat. No. 4,521,321), fatty amines (U.S. Pat. No. 4,839,067) and alpha olefin sulfonates (U.S. Pat. No. 4,604,220). It is also known to incorporate ingredients such as chelating agents, alcohols and low molecular weight glycols in order to improve the physical stability and operational characteristics of the lubricants.
It has been found that many prior aqueous lubricants can deleteriously affect certain types of thermoplastic materials such as PET, PBT (polybutylene terephthalate), polysulfones and polycarbonates, in that bottles or other articles formed of these materials are prone to stress cracking. Such stress cracking can lead to premature failure and leaking of the containers and is therefore a significant problem for beverage and food manufacturers. In particular, certain types of surfactants, alcohols, glycols and alkaline materials are known to promote stress cracking.
While a number of PET-compatible surfactants have been commercialized in the past (e.g., Dicolube PL.RTM. sold by the Diversey Corporation), these are generally less than optimum owing to cost or stress cracking problems. There is accordingly a need in the art for improved, low-cost conveyor lubricant which can be used with PET or other thermoplastic containers without fear of inducing significant stress cracking problems.